1. Field of the Invention
The present invention relates generally to the field of simulcast transmission systems, and more particularly to a simulcast system providing system clock synchronization and carrier frequency equalization.
2. Description of the Prior Art
The primary requirement for effective operation of simulcast transmission systems, such as used in simulcast paging systems, is to minimize the difference in audio phase delay in signals originating from two different transmission stations when received at the paging receiver. The audio phase delay can be minimized by requiring that different transmission stations transmit the same paging information at precisely the same point in time. Prior art paging systems have typically concentrated on equalizing the transmission path delay, including such elements as telephone lines, microwave links or RF links, which were used to connect the paging terminal to the transmission stations. In order to achieve such equalization of the transmission path delay, delay elements were introduced into the transmission path of those transmission stations closest to the source, or origin of the signal transmission, thereby providing a substantially uniform transmission path delay for all transmission stations throughout the system. Unfortunately, once such simulcast transmission systems were equalized, there was no guarantee the equalization would remain constant throughout any particular transmission period, because several of the transmission elements, particularly the telephone lines when they were not dedicated, were subject to variation throughout the transmission period.
In order to overcome the deficiencies noted above, several prior art simulcast transmission systems have utilized which has become known as a "store and forward" transmission technique, wherein the transmission data is stored at the individual transmission stations within the system and then broadcast, or forwarded, from all transmission stations at a predetermined time. Equalization of such systems have relied on the use of global positioning satellite systems which provided the accurate timing control necessary to control the timing of transmissions throughout the system. While such systems using global positioning satellites have proved effective in providing control of the transmission timing requirements, the advantages are provided at a substantial cost differential as compared to conventional simulcast transmission equalization systems.
There is a need to provide simulcast system equalization capability to without the use of a global positioning satellite system.